This invention relates to a nickel-hydrogen secondary battery.
As a nickel-hydrogen secondary battery where a hydrogen-absorbing alloy is employed as a negative electrode, those provided with a negative electrode constituted by an LaNi.sub.5 -based alloy having CaCu.sub.5 type crystal structure as a main phase, or by an alloy having, as main phase, a Laves phase containing constituent elements such as Ti, Zr, V and Ni have been conventionally put into practical use.
In particular, most of secondary batteries produced at present are occupied by a nickel-hydrogen secondary battery employing an LaNi.sub.5 -based hydrogen-absorbing alloy, and this nickel-hydrogen secondary battery has been extensively employed. However, since the hydrogen absorption capacity of the LaNi.sub.5 -based alloy is basically in the ratio of 1:1 (hydrogen atom: metal atom), it is very difficult with this LaNi.sub.5 -based alloy to enhance the hydrogen absorption over the aforementioned ratio. When this hydrogen absorption capacity of the LaNi.sub.5 -based alloy is converted into an electrochemical capacity, it corresponds to about 370 mAh/g. However, since the LaNi.sub.5 -based alloy which is currently employed in the nickel-hydrogen secondary battery already indicates a hydrogen absorption capacity of as high as about 330 mAh/g, a more prominent increase in capacity density can be hardly expected as far as this LaNi.sub.5 -based alloy is concerned.
On the other hand, the hydrogen-absorbing alloy containing the Laves phase as a main phase is known as being capable of absorbing hydrogen in the ratio of 1:1 or more (hydrogen atom:metal atom). Therefore, it is theoretically possible with this Laves phase type hydrogen-absorbing alloy to enhance the capacity density of hydrogen absorption. However, this Laves phase type alloy is accompanied with problems that a stable oxide film tends to be formed on the surface of the alloy thereby making it impossible to sufficiently utilize the alloy, that it takes a relatively long time for initially activating the alloy, that it is difficult to obtain a sufficient charge/discharge efficiency, and that it is difficult to achieve a high capacity in concurrent with other desired battery properties, thus making it difficult to utilize the Laves phase type alloy as a material for the negative electrode.
Meanwhile, a newly developed hydrogen-absorbing alloy comprising, as main constituent elements, magnesium, nickel and a rare earth element is featured in that the absorption capacity thereof both per volume and per weight is higher than that of the LaNi.sub.5 -based alloy, and also in that the activation velocity thereof is higher and the charge/discharge efficiency is more excellent as compared with the Laves phase type hydrogen-absorbing alloy. Therefore, it is possible with this hydrogen-absorbing alloy to realize a secondary battery exhibiting a higher capacity as compared with a secondary battery provided with a negative electrode comprising the LaNi.sub.5 -based alloy, and also exhibiting a more excellent charge/discharge efficiency as compared with a secondary battery provided with a negative electrode comprising the Laves phase type hydrogen-absorbing alloy.
However, if an alkaline solution about 6 to 8 in normality is employed as an electrolyte in an alkaline battery provided with a negative electrode comprising a hydrogen-absorbing alloy containing magnesium, nickel and a rare earth element as a main constituent element, the surface of the hydrogen-absorbing alloy constituting the negative electrode which is contacted with the electrolyte is caused to oxidize slightly. As a result, hydroxides or oxides of magnesium, a rare earth element, a transition metal such as nickel, cobalt and manganese that have been generated through the aforementioned oxidation are caused to diffuse into the voids of the separator while repeating the dissolution and precipitation thereof in the electrolyte due to the fluctuation in temperature resulting from the generation of Joule heat at the occasion of charging/discharging. Therefore, when the secondary battery is kept in an erosion-promoting environment such as a high temperature storage in particular, the self-discharge of battery is accelerated due to a lowering in resistance of the separator, thus raising a problem.
Meantime, International Re-publication No. WO97/03213 discloses a hydrogen-absorbing electrode comprising a hydrogen-absorbing alloy having a specific antiphase boundary and a composition represented by the following general formula. This hydrogen-absorbing alloy is mentioned therein as having an LaNi.sub.5 type crystal structure. EQU (R.sub.1-x L.sub.x)(Ni.sub.1-y M.sub.y).sub.z
wherein R is La, Ce, Pr or Nd; L is Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc, Mg or Ca; M is Co, Al, Mn, Fe, Cu, Zr, Ti, Mo, Si, V, Cr, Nb, Hf, Ta, W, B or C; and x, y and z are atomic ratio individually defined as 0.05.ltoreq.x.ltoreq.0.4, 0.ltoreq.y.ltoreq.0.5, and 3.0.ltoreq.z.ltoreq.4.5.